1. Field of the Invention
The present invention relates to an optical disc device and focus control method, and is preferably applied to for example an optical disc device that reproduces information from an optical disc that has separate layers: a reference layer and a recording layer. The emission position of the optical beam is determined according to the reference layer; the information is recorded on the recording layer.
2. Description of the Related Art
There is an optical disc device that reproduce information by emitting an optical beam toward an optical disc, such as Compact Disc (CD), Digital Versatile Disc (DVD) or “Blu-ray Disc (Registered Trademark: referred to as BD, hereinafter), and detecting the reflection. Such optical disc devices have been popular.
By emitting an optical beam to the optical disc, such an optical disc device actually changes its reflectance locally, thereby recording information.
As for the optical disc, the size of a beam spot formed when the optical beam is converged by components like an objective lens is approximately determined based on λ/NA (λ: the wavelength of the optical beam, NA: numerical aperture). Moreover, its resolution is proportional to that value. For example, the BD-type optical disc with 120 mm in diameter can record about 25 GB of data per layer.
By the way, the optical disc is designed to record various kinds of information, such as various kinds of content (including music and video content) and various kinds of computer data. The amount of information to be recorded has been growing due to high-definition video data or high-quality music data. Moreover, the number of pieces of content to be recorded on one optical disc has been increasing. Therefore, the optical disc is required to have a larger capacity.
To increase the capacity and simplify the structure of the optical disc, some optical disc devices make use of holograms: with a standing-wave recoding technique, they record information evenly on a plurality of recording layers (see Jpn. Pat. Laid-open Publication No. 2007-220206 (FIG. 24)).